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Publication Date

June 2001


The recently observed Flow-Assisted Corrosion (FAC) of CANDU outlet feeders is associated with a texturing of the corroding surface, called "scalloping", that imparts the appearance of orange peel. To study the evolution of erosion and dissolution patterns contributing to scalloping and to derive the link with corrosion, experiments on the dissolution of pipes made of plaster of Paris (CaSO4.½H2O) are being carried out. Flow-Assisted Corrosion is a significant problem with carbon steel components handling rapidly moving water or water/steam mixtures in the power generating industry. The mechanism of FAC in outlet feeders of CANDU reactors can be described in terms of a mixture of surface dissolution and erosion controlling the thickness of the protective oxide. Erosion is influenced by the combined action of flow-induced mechanical forces (shear stresses, pressure variations by high flow velocity and particle impact in multi-phase flows) and dissolution by electrochemical processes. The method chosen to study scalloping is the dissolution of pipes made of plaster of Paris (CaSO4.½H2O) in a recirculating water loop. The solubility of plaster of Paris in water is high enough (2.6 g/l) so that significant dissolution is obtained after a short time. The conductivity and the pH of the water flowing through the loop test section have been recorded and post-test examination has been carried out. Scallops have been observed along the plaster surface at the end of the tests. Their characteristics are strongly related to the flow rate. Scallop size decreases with increasing flow rate whereas surface density of scallops increases with increasing flow rate. It has been found that the scallop pattern develops at the upstream end of the pipe and moves downstream. The collected data are used to develop models describing the significance of scallops in FAC.


Flow-Assisted Corrosion, Fac, Scalloping, Carbon, Candu

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